1. Field of the Invention
The present invention relates to a stitching mode changer designed to automatically change stitching modes of an overlock machine from ordinary overlock stitching to roll heming and vice versa.
2. Description of the Related Art
In general, overlock machines are designed to operate in two selective stitching modes, as shown in FIGS. 8 and 9, that is, ordinary overlock stitching in which the edge of a fabric cut by a knife is stitched with the needle thread and the upper and lower looper threads, and roll hemming in which the edge of a fabric cut is enfolded with the roll hemming. The roll hemming includes a variant in which the tension applied to the lower looper thread is increased, and the enfolded edge of a cut fabric is stitched with the upper looper thread.
To change the stitching mode from the ordinary overlock stitching to the roll hemming, it has heretofore been necessary to execute the following operations in a proper sequence: (1) the stitch width adjusting member is removed or withdrawn from the needle plate; (2) the stitch width is changed by operating the stitch width adjusting dial (cutting width adjusting dial); and (3) the fabric feed is changed to about 1 mm by adjusting the fabric feed dial. To change the standard roll hemming to the special roll hemming, an operation (4) of adjusting the tension of the lower looper thread is added to the above-described three operations.
The prior art suffers, however, from some problems. That is, since the stitching mode changing operation is complicated and requires skill and time, the prior art is unsuitable for unexperienced users. Even experienced users are likely to make a mistake in operation. In particular, since the stitch width adjusting member cannot be stably fixed after being advanced toward the needle plate, it is likely that the lower looper and the adjusting member will collide with each over during the sewing operation, resulting in breakage of the parts.
To simplify and automate the stitching mode changing operation, an automatic stitching changer has been proposed in which a stitch width adjusting member (thread slide member) is withdrawn in response to the operation of an lower knife adjusting dial, and a stitch balancing thread tension sensor is also activated in response to the operation of the adjusting dial so as to change the bobbin thread tension (see Japanese Patent Application Laid-Open (KOKAI) No. 02-154788 (1990)). With this automatic stitching changer, the operations (1), (2) and (4) in the above-described procedure are executed by the adjusting operation of the lower knife adjusting dial, and then the fabric feed is adjusted.
However, the above-described automatic stitching changer necessitates adjustment of the fabric feed as an extra operation. For this reason, if the user forgets to adjust the fabric feed to a level suitable for the ordinary overlock stitching after changing the roll hemming mode to the ordinary overlock stitching mode, since the configuration of the thread slide portion of the adjusting member is not suitable for small pitches in general, the forward movement of the fabric is retarded, so that the fabric may cause jamming. If the needle sticks in the fabric jammed in this way, the needle may break, and other problems may also arise, i.e., breakage of the thread slide of the fabric presser or the upper and lower loopers.